1. Field of the Invention
The present invention relates to a method of oscillating a spot of a laser beam incident upon a workpiece for processing the workpiece, and a laser processing system adapted to practice the method and equipped with a laser beam oscillating apparatus for oscillating the laser beam spot.
2. Discussion of the Related Art
A laser processing system is known, wherein a workpiece is irradiated with a laser beam, to perform various processing operations such as welding, severing or cutting, hardening, and surface treatment or finishing which may involve removal of a material from the surface of the workpiece. Such laser processing system uses a high-energy laser beam which impinges a desired local portion of the workpiece and which permits local concentration of a laser beam for precision machining or other processing in a relatively small area of the workpiece with a comparatively high degree of accuracy. In some processing operations, a local portion of the workpiece to be processed extends in a given direction and has a predetermined width in the direction perpendicular to the direction of extension. In this case, the spot of the laser beam (hereinafter referred to as "beam spot") is moved in the direction of extension of the local portion to be processed, while at the same time the beam spot is oscillated at a predetermined frequency in the direction of width of the local portion. The oscillation of the beam spot takes the form of either reciprocating movements in a direction substantially parallel to the direction of width of the local portion, or rotating movements in a merry-go-round fashion within the width of the local portion. Where the local portion of the workpiece to be processed is curved, arcuate or annular, for example, the direction of the width of such curved, arcuate or annular local portion varies in the direction of extension, and the oscillating direction of the beam spot must be changed as the beam spot moves in the direction of extension. Accordingly, the laser processing system must be equipped with a laser beam oscillating apparatus capable of changing the oscillating direction of the beam spot on the workpiece as the processing progresses in the direction of extension of the local portion to be processed.
An example of laser beam oscillating apparatus of reciprocatory oscillation type is shown generally at 200 in FIG. 19. This apparatus 200 uses an X-axis oscillator mechanism 210 and a Y-axis oscillator mechanism 212 which include respective movable reflecting mirrors 202, 204 that are rotatable about respective axes. The reflecting mirrors 202, 204 are rotated by respective electromagnetic actuators 206, 208 about their axes as indicated by arrows in FIG. 19. The X-axis and Y-axis oscillator mechanisms 210, 212 are positioned relative to each other such that the axes of the two mirrors 202, 204 intersect each other.
An example of laser beam oscillating apparatus of rotary oscillation type is known as disclosed in JP-A-3-285785 (published on Dec. 16, 1991) wherein a condensing lens whose optical axis is offset from the axis of the laser beam is rotated about the optical axis, to rotate the beam spot.
The known laser beam oscillating apparatuses indicated above suffer from various drawbacks as described below. In the reciprocatory oscillation type apparatus of FIG. 19, the oscillating direction in which the beam spot is reciprocated is determined by cooperative operations of the X-axis and Y-axis oscillator mechanisms 210, 212. This means relatively complicated and difficult control of the two oscillator mechanisms to change the oscillating direction. Further, the spacial positioning of the two oscillator mechanisms requires a relatively large space for installation of the apparatus. Another drawback of the apparatus is a need of using a condensing convex lens having a relatively large focal length, since the laser beam L condensed by the condensing lens must be incident upon the two oscillator mechanisms disposed downstream of the condensing lens. In this respect, the light condensing efficiency is undesirably low in the apparatus of FIG. 19.
The laser beam oscillating apparatus of rotary oscillation type disclosed in the publication JP-A-3-285785 is disadvantageous in that the application of the apparatus is limited due to the rotating movements of the beam spot, that the apparatus cannot be used with a high-energy laser source since it is difficult to effectively cool the condensing lens rotated at a high speed, and that the light condensing device must use a lens and cannot use a metallic surface mirror, leading to comparatively short life expectancy and increased running cost.